INVERTEBRATE PALJ30NIOLOGY. 201 



which two pits arc for the reception of the cardinal teeth of the right valve; 

 anterior and posterior lateral teeth both elongated parallel to the cardinal 

 margin, the former approaching more nearly to the cardinal teeth. Right 

 valve with, under the apex, two diverging, slender, cardinal teeth, like the 

 posterior one of the other valve, with a triangular pit between them for the 

 reception of the principal cardinal tooth of the left valve; anterior one more 

 oblique than the other, and nearly or quite connecting with the lateral tooth 

 on that side; lateral teeth like those of the left valve; the anterior one 

 apparently fitting under and the posterior above that of the other valve. 

 Muscular impressions shallow ; pallial impression with a moderate rounded 

 sinus, directed obliquely forward and upward. Ligament unknown, but be- 

 lieved to be external. Surface without ornamentation. 



After searching in vain to find some defined group under which I could 

 range this shell, with a reasonable degree of probability of its properly belong- 

 ing to the same, I concluded to propose a new genus for its reception It 

 seems to be somewhat related to Arcopagia, or, at least, to forms that have 

 been referred to that genus, but differs in having anterior and posterior lateral 

 teeth in both valves, as well as in the form of the anterior cardinal tooth of 

 the left valve and its arrangement with relation to the other. At first, I was 

 inclined to think it might find a place in Mr. Conrad's Cretaceous genus 

 Tellinimera ; but a sketch of the hinge of his type, received from him, shows 

 the latter to be quite distinct, having a double anterior lateral tooth in the 

 right valve, and the cardinal teeth of the same consisting of one bifid, very 

 oblique, tooth, and immediately behind this a minute, vertical second cardinal. 

 The hinge-plate of his genus is also distinctly wider on the anterior side than 

 in the type under consideration. 



It is somewhat remarkable that the hinge-structure of the type of this 

 genus is, in most respects, very similar to that of the group for which I have 

 proposed the name Crassatellina, while the whole aspect of the two shells is 

 so entirely different that I can scarcely doubt that they really belong to two 

 distinct families : Crassatellhia probably belonging to the Crasmtel/idce ; and 

 Arcopagella possibly to the Tellinidce, or some allied family. As closely as 

 their hinges resemble, however, a critical comparison of the accompanying 

 figures will show them to present important differences of details. 



It is probable that this genus will be found to include other Tellinoid 



Cretaceous shells, the hinges of which are yet unknown. 

 26 ii 



